Various technologies have been developed for applying thin films over substrates, and particularly for applying thin films during fabrication of semiconductor devices. Such technologies include chemical vapor deposition (CVD) and atomic layer deposition (ALD). ALD and CVD are similar to one another in that both comprise utilization of volatile precursor materials to form a desired deposit over a substrate. CVD and ALD differ from one another, however, in that CVD typically includes reaction of precursors in vapor phase over a substrate to form a desired deposit, whereas ALD typically comprises chemisorption of a precursor component onto a substrate followed by reaction with the chemisorbed component to form a desired deposit.
Specific attributes of typical ALD technology and typical CVD technology are described below. First, however, it is useful to provide definitions of particular terms utilized throughout this document. The deposition methods referred to herein can be described in the context of formation of materials on one or more semiconductor substrates. In the context of this document, the term “semiconductor substrate” or “semiconductive substrate” is defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate”refers to any supporting structure, including, but not limited to, the semiconductive substrates described above. Also in the context of the present document, “metal” or “metal element” refers to the elements of Groups IA, IIA, and IB to VIIIB of the periodic table of the elements (i.e., groups 1-12 of the new IUPAC system) along with the portions of Groups IIIA to VIA (groups 13 and 14 of the new IUPAC system) designated as metals in the periodic table, namely, Al, Ga, In, Tl, Ge, Sn, Pb, Sb, Bi, and Po. The lanthanides and actinides are included as part of Group IIIB; and the “lanthanides” and “actinides” are to be understood to include lanthanum and actinium, respectively. “Non-metals” refers to the remaining elements of the periodic table.
Next, referring to attributes of ALD technology, such can, but does not always, involve formation of successive atomic layers on a substrate. The layers may comprise, for example, an epitaxial, polycrystalline, and/or amorphous material. ALD may also be referred to as atomic layer epitaxy, atomic layer processing, etc.
Described in summary, ALD includes exposing an initial substrate to a first chemical species to accomplish chemisorption of the species onto the substrate. Theoretically, the chemisorption forms a monolayer that is uniformly one atom or molecule thick on the entire exposed initial substrate. In other words, a saturated monolayer. Practically, as further described below, chemisorption might not occur on all portions of the substrate. Nevertheless, such an imperfect monolayer is still a monolayer in the context of this document. In many applications, merely a substantially saturated monolayer may be suitable. A substantially saturated monolayer is one that will still yield a deposited layer exhibiting the quality and/or properties desired for such layer.
The first species is purged from over the substrate and a second chemical species is provided to chemisorb onto the first monolayer of the first species. The second species is then purged and the steps of utilizing the first and second species can be repeated with exposure of the second species monolayer to the first species. In some cases, the two monolayers may be of the same species. Also, a third species or more may be successively chemisorbed and purged just as described for the first and second species. It is noted that one or more of the first, second and third species can be mixed with inert gas to speed up pressure saturation within a reaction chamber.
Purging may involve a variety of techniques including, but not limited to, contacting the substrate and/or monolayer with a carrier gas and/or lowering pressure to below the deposition pressure to reduce the concentration of a species contacting the substrate and/or chemisorbed species. Examples of carrier gases include N2, Ar, He, Ne, Kr, Xe, etc. Purging may instead include contacting the substrate and/or monolayer with any gaseous substance that allows chemisorption byproducts to desorb and reduces the concentration of a species preparatory to introducing another species. A suitable amount of purging can be determined experimentally as known to those skilled in the art. Purging time may be successively reduced to a purge time that yields an increase in film growth rate. The increase in film growth rate might be an indication of a change to a non-ALD process regime and may be used to establish a purge time limit.
ALD is often described as a self-limiting process, in that a finite number of sites exist on a substrate to which the first species may form chemical bonds. The second species might only bond to the first species and thus may also be self-limiting. Once all of the finite number of sites on a substrate are bonded with a first species, the first species will often not bond to other of the first species already bonded with the substrate. However, process conditions can be varied in ALD to promote such bonding and render ALD not self-limiting. Accordingly, ALD may also encompass a species forming other than one monolayer at a time by stacking of a species, forming a layer more than one atom or molecule thick. The various aspects of the present invention described herein are applicable to any circumstance where ALD may be desired. It is further noted that local chemical interactions can occur during ALD (for instance, an incoming reactant molecule can displace a molecule from an existing surface rather than forming a monolayer over the surface).
Traditional ALD can occur within frequently-used ranges of temperature and pressure, and according to established purging criteria to achieve the desired formation of an overall ALD layer one monolayer at a time. Even so, ALD conditions can vary greatly depending on the particular precursors, layer composition, deposition equipment, and other factors according to criteria known by those skilled in the art.
Referring next to CVD technology, such includes a variety of more specific processes, including, but not limited to, plasma enhanced CVD and others. CVD is commonly used to form non-selectively a complete, deposited material on a substrate. One characteristic of CVD is the simultaneous presence of multiple species in the deposition chamber that react to form the deposited material. Such condition is contrasted with the purging criteria for traditional ALD wherein a substrate is contacted with a single deposition species that chemisorbs to a substrate or previously deposited species. In addition, an ALD process regime may provide a simultaneously contacted plurality of species of a type or under conditions such that ALD chemisorption, rather than CVD reaction occurs. Instead of reacting together, the species may chemisorb to a substrate or previously deposited species, providing a surface onto which subsequent species may next chemisorb to form a complete layer of desired material.
Under most CVD conditions, deposition occurs largely independent of the composition or surface properties of an underlying substrate. By contrast, chemisorption rate in ALD might be influenced by the composition, crystalline structure, and other properties of a substrate or chemisorbed species.
Among the advantages of ALD-type technologies (with the term “ALD-type” referring to technologies that are either true ALD processes or that are more similar to ALD processes than to other deposition processes), is that such can theoretically be self-limiting processes. Specifically, a substrate exposed to appropriate precursor will only have a monolayer chemisorbed thereover, regardless of the length of time of the exposure or the quantity of precursor utilized in the exposure. In other words, the substrate can be exposed to an excess of precursor, and yet only a monolayer will be formed.
As another aspect of the prior art, it is desired to fabricate various dielectric materials to have a high dielectric constant (in other words, a high k value). Such high-k dielectric materials can be utilized in, for example, capacitors as capacitor dielectrics. Among the compositions suitable for incorporation into high-k dielectric materials is hafnium oxide. The hafnium oxide can exist in numerous forms, including an amorphous form, and monoclinic, tetragonal, cubic and orthorhombic crystalline forms. The preferred form of hafnium oxide for high-k dielectric materials is the tetragonal crystalline form, as such has the highest dielectric constant of the various forms of hafnium oxide. It is thus desirable to develop methodologies for controllably forming hafnium oxide having tetragonal crystalline structure throughout.
Zirconium oxide has similar properties to hafnium oxide, and it would be desirable if the methodologies developed for formation of hafnium oxide could also be utilized for deposition of zirconium oxide.
It could also be desirable to for new methodologies to be applicable for deposition of other materials, in addition to, or alternatively to, hafnium oxide and zirconium oxide.